A-priori analysis of subgrid scalar flux models for turbulent high Schmidt number passive scalar mixing

Numerical study of high Schmidt (Sc) number scalars using LES is hampered by the fact that few well-tested models exist specifically for this regime. Low Sc LES studies generally employ the widely used eddy-viscosity type models. However, these suffer from the fact that the resolved scalar gradient is often not well-aligned with the subgrid scalar flux (SGF). In this work, we perform an a-priori analysis based on Sc$\gg$1 simulations to explore the alignment of the SGF with the resolved as well as subgrid velocity and scalar quantities. Special attention is given to differences observed in the inertial-convective and the viscous-convective sub-ranges. These differences are related to the fundamental dissimilarities in the transport mechanisms. The alignment trends are used to ascertain the suitability of existing models for use in both sub-ranges.